Modern motor vehicle brake systems, designed in order to execute an electronic stability program (ESP) or traction controlling, standardly include a hydraulic pump that is controlled by a control device. In a critical driving situation, the hydraulic pump is automatically controlled, and increases the braking pressure acting on the wheel brake, suctioning brake fluid from the brake fluid reservoir or from a low-pressure storage device. The braking pressure at the wheel is, as a rule, modulated by actuating an intake valve and an outlet valve, while the hydraulic pump runs with a predetermined nominal power and, in combination with a changeover valve, sets the system to a system pressure.
FIG. 1 shows, as an example, a hydraulic brake system 17 known from the prior art that is designed for the execution of an electronic stability program. Brake system 17 includes two brake circuits 19a, 19b in an X-arrangement that are fashioned symmetrically. In the following, reference is therefore made only to part 19a, shown at the left in the Figure.
Brake system 17 includes a foot brake pedal 1, a brake servo (power brake unit) 2 with a brake master cylinder 4 connected thereto, on which a brake fluid reservoir 3 is situated. When foot brake pedal 1 is actuated, a corresponding pressure is produced in main brake lines 5a, 5b, which acts on wheel brakes 11 of the right front wheel (RF) and of the left rear wheel (LR) via a changeover valve 8a and the two intake valves 10a, 10b. The path in which pressure builds up when foot brake pedal 1 is actuated is identified by arrows b. In this basic state (state without flow), changeover valve 8a is open and a high-pressure switching valve 7a situated parallel thereto is closed. Changeover valve 8a is used to switch back and forth between foot pedal operation and an automatic control operation in which there is active pressure buildup by the system itself.
When there is an intervention of the electronic stability program, hydraulic pump 9a, or its motor 16, is controlled by a control device 18, and automatically builds up braking pressure that acts on wheel brakes 11. In this control state, changeover valve 8a is controlled and high-pressure switching valve 7a, which is situated between brake master cylinder 4 and hydraulic pump 9a, is opened. Hydraulic pump 9a then conveys the hydraulic fluid along paths a to wheel brakes 11. The hydraulic fluid thus flows from brake fluid reservoir 3 through main brake line 5a, high-pressure switching valve 7a, a suction line 6a, through hydraulic pump 9a, and further through intake valves 10a, 10b, into wheel brakes 11. In order to modulate the braking pressure, intake valves 10a, 10b (function: “hold pressure”) and outlet valves 13a, 13b (function: “decrease pressure”) are correspondingly opened or closed. In the “decrease pressure” state, the hydraulic fluid flows through outlet valves 13a, 13b into a low-pressure storage device 14a, from which hydraulic pump 9a suctions the hydraulic fluid and pumps it back into the system pressure circuit, or to wheel brakes 11.
A rapid reaction of the electronic stability program system is possible in particular if, at the beginning of a controlling action, low-pressure storage device 14 contains a sufficient quantity of hydraulic fluid that can be suctioned by hydraulic pump 9a. In order to ensure, before a controlling action, a sufficient filling state of low-pressure storage device 16a, it is known to partially fill low-pressure storage device 16a (pre-filling). For this purpose, hydraulic pump 9a is controlled and valves 7a and 13a are opened, and in this way hydraulic fluid is pumped into low-pressure storage device 14a. In this process, the operation of hydraulic pump 9a and the actuation of valves 7a and 13a produces a relatively loud noise that has a disturbing effect and that can irritate the driver.
It is therefore an object of the present invention to provide a method for pre-filling the low-pressure storage device, or to provide a correspondingly designed brake system in which the hydraulic pump need not be actuated in order to pre-fill the storage device, so that no disturbing noises arise.